1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use as, for example, a band pass filter. More specifically, the present invention relates to a structure of a surface acoustic wave filter (SAW filter), in which a plurality of one-port surface acoustic wave resonators (SAW resonators) are connected so as to define a ladder circuit, and a method of manufacturing thereof.
2. Description of the Background Art
Conventionally, a SAW filter has been widely used as a band pass filter. For example, in Japanese Examined Patent Publication No. 56-19765, there is provided a SAW filter having an arrangement such that a plurality of one-port SAW resonators are arranged to constitute a ladder circuit.
Referring to FIGS. 11 and 12, a description of the above-mentioned SAW filter having a ladder circuit structure will be provided. In the SAW filter of FIGS. 11 and 12, a series arm for connecting an input end and an output end, and a parallel arm for connecting the series arm and a reference potential are provided. A one-port SAW resonator S1 defining a series-arm resonator is connected to the series arm, and a one-port SAW resonator P1 defining a parallel-arm resonator is connected to the parallel arm. In FIG. 11, only one series-arm resonator and one parallel-arm resonator are shown. However, the number of series-arm resonators and parallel-arm resonators included in the filter is determined by the desired filter characteristics.
Referring to FIG. 12, the conventional one-port SAW resonator has an electrode structure such that an IDT 51 has a reflector 52 on a first side thereof and a reflector 53 on a second side thereof, all arranged on a piezoelectric substrate (not shown).
The IDT 51 has a pair of bus bars 54 and 55 which extend along a direction in which a surface acoustic wave propagates. The bus bar 54 is connected to one end of each of a plurality of electrode fingers 56. The electrode fingers 56 extend in a direction that is perpendicular to the direction in which a surface acoustic wave propagates, in other words, towards the bus bar 55 on the opposite side of the bus bar 54. Similarly, the bus bar 55 is connected to one end of each of a plurality of electrode fingers 57. The electrode fingers 57 extend towards the bus bar 54. The electrode fingers 56 and 57 are arranged to be interdigitated with each other.
A plurality of the above one-port SAW resonators are arranged to constitute the ladder circuit as shown in FIG. 11, so as to define a SAW filter. FIG. 13 shows the attenuation-frequency characteristics of the SAW filter.
Since the SAW filter having the ladder circuit structure exhibits small loss and has a wide pass band, SAW filters have been widely used as band pass filters in cellular phones or other similar devices.
However, in recent years, cellular phones have been using a system in which a transmission-side frequency band and a reception-side frequency band are close to each other. Thus, band pass filters should now be able to increase the steepness of the filter characteristics when pass bands are close to each other.
Therefore, in order to increase the steepness of the filter characteristics, in Japanese Unexamined Patent Publication No. 9-167937, there is provided a SAW filter having a circuit structure as shown in FIG. 14. In this case, one-port SAW resonators S1 and S2 are connected to a series arm such that the SAW resonator S1 is connected in parallel to a capacitor 58. According to this conventional device, the addition of the capacitor 58 permits the anti-resonance frequency of the SAW resonator S1 to be decreased so that the steepness of the filter characteristics on the high-frequency side of the pass band can be increased.
However, since the method described in Japanese Unexamined Patent Publication No. 9-167937 requires the addition of the capacitor 58, the size of the SAW filter is increased, and thus, the conventional design is difficult to use if miniaturization of cellular phones or other similar devices is desired. Additionally, since more capacitance is added due to the capacitor 58 to the SAW resonator S1, the attenuation within frequency bands other than the pass band is reduced.
Note that in a SAW filter having the ladder circuit structure in which frequency bands are spaced from the pass band, attenuation is determined by the capacitance ratio of a parallel-arm resonator and a series-arm resonator. In general, when the capacitance of the series-arm resonator is increased, the attenuation is decreased. Consequently, as described above with respect to the conventional device, when the series-arm resonator S1 is connected in parallel to the capacitor 58, the attenuation outside of the pass band is deteriorated, as in the case in which the capacitance of the series-arm resonator is increased.
In order to prevent the above-described decrease in the attenuation within the frequency bands which are spaced away from the pass band, a solution is to reduce the capacitance of the electrode of the series-arm resonator. However, when the capacitance of the electrode of the series-arm resonator is reduced, it is necessary to reduce the number of pairs of electrode fingers and the interdigitating width thereof, which leads to difficulties in obtaining desirable resonance characteristics.